One aspect relates to power semiconductor arrangements including a power semiconductor module mechanically connected to a heat sink.
Power semiconductor modules may include a base plate and at least one, at least two, or more semiconductor chips that are generally arranged on a circuit carrier. One or more circuit carriers equipped in this way are in turn fixedly connected to the baseplate. To form a power semiconductor arrangement, the power semiconductor module may be mechanically pressed against a heat sink with the baseplate ahead.
An exemplary embodiment of such a power semiconductor module is illustrated in FIG. 1. The power semiconductor module 1 exhibits a semiconductor chip 6 including a semiconductor body 60 provided with metallizations 61, 62 on two opposite sides. The first metallization 61 is generally structured and divided at least into control and load current contacts. Furthermore, the power semiconductor module 1 includes a circuit carrier 5 including a dielectric carrier 50 provided with a structured first metallization layer 51 on one side and with a second metallization layer 52 on the opposite side.
The semiconductor chip 6 is connected by its metallization 62 to a portion of the structured metallization 51 of the circuit carrier 5 by means of an electrically conductive connecting layer 71, for example, a solder layer or an NTC layer (NTC=low-temperature connection).
Furthermore, the first metallization 61 of the semiconductor chip 6 is connected to a further portion of the structured metallization layer 51 of the circuit carrier 5 by means of bonding wires 7. As an alternative to a bonding connection, it is also possible to provide metal clips, for example, which are connected to the first metallization 61 by means of a soldering or low-temperature connection.
The circuit carrier 5 serves, on the one hand, for electrically interconnecting one or more semiconductor chips 6 arranged thereon, and, on the other hand, the heat loss arising in the semiconductor chip 6 is dissipated via it. This results in the necessity of using materials including good thermal conductivity for the circuit carrier 5.
In order to increase the thermal cycling stability of the first solder layer 71, the coefficients of thermal expansion of the circuit carrier 5 and of the semiconductor chip 6 differ as little as possible. Since the coefficient of thermal expansion of the semiconductor chip 6 is essentially determined by the low coefficient of thermal expansion of the semiconductor body 60, DCB substrates (DCB=Direct Copper Bonding), DAB substrates (DAB=Direct Aluminum Bonding) or AMB substrates (AMB=Active Metal Brazed) are usually used as circuit carrier 5 since they likewise include a low coefficient of thermal expansion.
Two or more circuit carriers 5 equipped in this way are fixedly connected to a baseplate 2 by means of a second connecting layer 72 at their second metallization layer 52. Baseplates of this type may, for example, wholly or partly include a metal such as, for example, copper or aluminum and therefore include a coefficient of thermal expansion that differs comparatively greatly from the coefficient of thermal expansion of the circuit carrier 5.
Since the heat arising in the semiconductor chip 6 is dissipated via the circuit carrier 5, the second connecting layer 72 and the baseplate 2 to a heat sink 4 that may be connected to the power semiconductor module 1, during operation of the power semiconductor module 1 in the case of frequent temperature changes severe thermal cycling loading occurs in the second connecting layer 72.
On account of the different coefficients of the thermal expansion of the baseplate 2 and of the circuit carrier 5, von-Mises stresses act in the second connecting layer 72 and load the second connecting layer 72. Cracking can thereby commence in the second connecting layer 72 depending on the number of temperature cycles undergone and on the temperature differences that occur, whereby the heat dissipation from the semiconductor chip 6 in the direction of the heat sink 4 is impaired. This in turn brings about an increase in the temperature in the semiconductor chip 6, whereby the second connecting layer 72 is loaded to an even greater extent.
Further loading of the second connecting layer 72 arises as a result of mechanical stresses which occur at fixing locations 3 for fixing the baseplate 2 and hence the power semiconductor module 1 at a heat sink 4.
FIG. 2 illustrates a perspective view of a quarter model of a baseplate 2, which is equipped with 3 circuit carriers in this example, to which baseplate a circuit carrier 5 is fixedly connected by means of a connecting layer 72.
A fixing location 3 formed as a continuous opening is provided in the region of an outer corner of the baseplate 2, by means of which fixing location the baseplate 2 may be fixed at a heat sink.
What is problematic in this case is the stability of the connecting layer 72 in its portions 72a located in the region of the outer corners 5a of the circuit carriers 5. In this case, an outer corner 5a of the circuit carrier 5 is understood to mean a corner adjacent to which no other circuit carrier is arranged. Likewise problematic is the thermal cycling stability of the second connecting layer 72 in its portions 72b arranged in the region of the inner corners 5b of the circuit carrier 5. In this case, an inner corner 5b of the circuit carrier 5 is understood to mean a corner in the vicinity of which another circuit carrier 5 is arranged.
FIGS. 3A and 3B illustrate the von-Mises stresses—on that side of the second connecting layer 72 in accordance with FIGS. 1 and 2 which faces the baseplate 2 (FIG. 3A) and the substrate 5 (FIG. 3B).
It can be seen therefrom that peaks of the von-Mises stresses occur below the outer corners 5a in regions 72a of the second connecting layer 72 and below the inner corners 5b in regions 72b of the second connecting layer 72.
On account of the cracking associated therewith, progressive delamination of the circuit carrier 5 from the baseplate 2 occurs during operation of the power semiconductor module 1. The delamination begins in the region of the portions 72a, 72b of the second connecting layer 72 owing to the peak values of the von-Mises stresses that occur there.
FIG. 4 illustrates such delamination effects on the basis of ultrasound examinations using the example of a power semiconductor module onto whose baseplate four circuit carriers are soldered, after 200 (FIG. 4A), 1000 (FIG. 4B), 2000 (FIG. 4C) and 4000 (FIG. 4D) temperature cycles. The locations at which the delamination occurs are represented dark in FIGS. 4A to 4D and identified by the reference symbol 72c. 
FIG. 4 reveals that the delamination commences in the corner regions 72a, 72b (see FIG. 4B) and propagates toward the center of the respective circuit carriers as the number of temperature cycles increases.
FIG. 5 illustrates the profile of the von-Mises stresses σ on that side of the second connecting layer which faces the baseplate in the region of an outer corner 72a, at which the von-Mises stresses including a maximum value of 26.7 MPa.
FIG. 6 illustrates a cross section through a power semiconductor module in the region of the interface between the second metallization layer 52 of the dielectric carrier and the second connecting layer 72 in cross section. It can be seen therefrom that the second connecting layer 72 includes a crack 72c in the region of its side facing the second metallization layer 52.